This invention relates to an infrared optical fiber capable of transmitting a high-output laser light, particularly a laser light in the infrared region, with a small loss and to a method for the manufacture of the optical fiber.
In recent years, there have been great advances in the research and development of machines and equipment making use of laser lights. In industrial fields, laser lights have found widespread acceptance in optical communication and the manufacture of various shaped articles. In the field of medical instruments, adoption of laser lights as optical scalpels or for eliminating skin blemishes has already been realized. Of the various forms of lasers, the CO.sub.2 laser has a very large proportion of light energy in the infrared region of about 10.mu.. This wavelength falls in the range of infrared absorption by water. This CO.sub.2 laser, therefore, is highly effective in cutting water-containing materials such as paper and fabrics and living tissue. Further, in the semiconductor industry, the CO.sub.2 laser is used such as for silane-gas thermal decomposition during the annealing of silicon substrates and the formation of LSI circuits. In the field of office machines, it is utilized as a high-speed printing means. In these applied devices, the laser light is generally led to the objects under treatment by means of reflecting mirrors. In these applications, therefore, if a device is developed for transmitting infrared laser lights via a flexible fiber instead of reflecting mirrors, the device can be expected to simplify various laser machines and facilitate their operation. In the meantime, there has appeared a general trend toward adopting increasingly greater wavelengths in optical communication. Also in this area the development of an infrared optical fiber is strongly needed. From the practical point of view, however, manufacture of an infrared optical fiber capable of safely transmitting high-energy infrared laser lights is extremely difficult.
Assumed, for example, that a step index structure optical fiber of glass material such as quartz glass, hign-silica-content glass or soda lime glass which has already been established in optical communication technology is adopted as a medium for the transmission of infrared laser lights. The light transmissivity of such glass type optical fiber is, however, limited to infrared rays with wavelengths up to 2 or 3 .mu.m. On the other hand, when the infrared ray to be passed through the optical fiber possesses a high power, it is apt to be desirable to fix the wavelength in the neighborhood of 1.5 .mu.m in order to prevent heat buildup and burn-out due to the absorption loss of the optical fiber and to assure the expected efficiency of energy output from the optical fiber. This means, in short, that the CO.sub.2 laser cannot be applied to this kind of optical fiber.
The step index structure optical fiber made of an infrared transmitting substance such as a metal halogenide or arsenic selenium glass is capable of transmitting infrared rays of a greater wavelength on the order of 10 .mu.m with high efficiency. It is nevertheless extremely difficult to manufacture an optical fiber of this kind having a practical capacity tolerance. Since the infrared transmitting substance mentioned above is seriously deprived of its viscosity at its melting point, the fabrication technique of fibers (for fabrication of fibers with a preform at the melting point until a prescribed diameter is obtained) which is adopted generally in the production of glass fibers for communication is no longer capable of forming a smooth interface between the core and the clad, making the manufacture of a step index structure optical fiber impossible.
To overcome this difficulty, there has been tried a method which comprises preparing a preform (matrix) of a core material of a metal halogenide suitable for infrared optical transmission and forcibly extruding this preform through the orifice of a die at a temperature below the melting point thereby obtaining an optical fiber of a prescribed diameter. Since the softened preform is passed through the orifice of the die under application of pressure, the inner pressure generated in the extruded fiber causes irregular flow deformation in the radial direction. When this method is used in the preform of a step index structure, the slidability of the interface between the core and the clad is impaired. In the optical fiber wherein the interface is disturbed as described above, the efficiency of light transmission is degraded. Besides, the light which is scattered by the irregular interface may deviate from the boundary of the fiber and, in an extreme case, burn the clad on the fiber.
There has been conceived the idea of extrusion molding a core alone and subsequently inserting this core into a cladding material separately extrusion molded thereby producing a clad core type infrared optical fiber. In this case, the number of steps involved in the manufacture is increased, lowering the efficiency of the production operation. Moreover, it is very difficult to insert a core of a very small diameter on the order of several microns accurately within a clad and to continue this insertion of the core without the friction between the core and the clad causing any harm to the surface of the core. When the core surface sustains scratches due to the friction, then the produced clad core suffers from the same scattering loss as described above.
For the reasons described above, the infrared optical fiber made of an infrared transmitting substance such as a metal halogenide or arsenic selenium glass is impracticable despite its theoretical possibility. As means for the transmission of high-power infrared laser lights such as those produced by carbon dioxide gas laser, therefore, an optical system making effective use of reflecting mirrors has so far been adopted.